LEDs (Light Emitting Diode) are semiconductor electronic components capable of emitting light, and such electronic components can only emit red light having low luminosity in the early stage. With the continuous improvements of technology, the electronic components have been developed to emit visible light, infrared light and ultraviolet light, and there are also great improvements in luminosity. LEDs have advantages that cannot be incomparable by traditional light sources, such as high efficiency, long life, low damage possibility, high switching speed and high reliability, and have been widely used in fields of indicator lights, displays and lighting.
Under normal circumstances, the overall efficiency of a single-segment linear LED driver is determined by a turn-on voltage and an input voltage of an LED, which meets the following relationship:
  Eff  =            V      LED              V      IN      
FIG. 1 illustrates a common structure of a single-segment linear LED driver, wherein an AC voltage is converted into an input voltage VIN through a rectifier bridge and supplies power for an LED light segment; the LED light segment consists of n LED lights connected in series; an output terminal of the LED segment is connected with a constant current control chip, and the constant current control is realized through the switching of a constant current control transistor in the constant current control chip; a capacitor C and a resistor R are connected in parallel with the two terminals of the input voltage to serve as adjustable devices. Since the number of LEDs connected in series is fixed, the excess voltage when the input voltage exceeds a forward voltage drop is borne by the constant current control transistor connected to the LEDs. VIN-VLED is a voltage on an adjusting transistor. The higher the input voltage is, the lower the efficiency of a system will be.
Generally, in a single-segment linear LED driver, an output voltage can be increased by adding the number of LEDs, such that the turn-on voltage of the LED light segment is as close as possible to the input voltage, thereby increasing efficiency. However, it may bring about the problem that the range of the input voltage is relatively narrow, and meanwhile the efficiency is still relatively low in the case of a high input voltage.
In addition, a HIGH-VOLTAGE DROP CURRENT TECHNOLOGY during which the current should be reduced if the voltage is excessively high can be used to reduce the loss caused by a high voltage, but the constant current effect is not good, and efficiency promotion is also limited.
Therefore, how to solve the problem of narrow input voltage range and low efficiency in a single-segment linear LED driver has become one of the problems to be urgently solved by those skilled in the art.